The present invention concerns a breaker switch arranged on at least one connection means of an electric power supply or consumer device contained in a container whose internal operating temperature must not exceed a determined critical value for safety reasons.
The invention concerns in particular power supplies such as batteries and accumulators, designated hereinafter by the generic term xe2x80x9cbatteryxe2x80x9d whether the power supply is rechargeable or not, said batteries being provided with such a breaker switch which is sensitive to an abnormal rise in temperature.
The invention concerns more particularly such batteries of the lithium-ion type, of generally prismatic shape and having a high power density, a feature which is sought after for increasingly widely used portable devices such as mobile telephones, video cameras and other electronic devices. The working of such batteries is however delicate which can present risks for the user or for the contiguous electronic circuits. Indeed, when the battery is in a closed circuit with a strong current demand, during the charging phase for the accumulators, or during a malfunction due for example to improper use or storage at too high a temperature, the chemical reactions which are generated can cause partial decomposition of the electrolyte with generation of a gaseous release leading to and increase in pressure inside the battery container and a rise in temperature able to cause an explosion, inflammation, and damage to the contiguous electronic circuits.
In order to overcome this drawback, such batteries have to be fitted with safety devices causing a reduction in the internal pressure by means of a vent, a reduction in temperature and/or switching off the battery via a breaker switch. For batteries used in portable devices, one cannot envisage incorporating a heat exchanger able to control a rise in temperature as a safety device.
By way of example of a vent safety device, the device disclosed in European Patent No. 0 554 535, wherein an increase in internal pressure causes the opening of a valve which is normally kept closed by the pressure of an elastic washer, may be cited.
In European Patent Application No. 98105760, in the name of the Applicant and incorporated in the present application by reference, a particular arrangement of a thermofusible composition around a terminal of the battery allows a vent to be opened when the temperature reaches an abnormally high value.
By way of example of a breaker switch, U.S. Pat. No. 4,943,497, which in a way constitutes an improvement to European Patent No. 0 554 535, in that the increase in internal pressure in the battery also causes breakage of the connection means between a terminal and the active material within the battery, may be cited. Devices of this type have the drawback of relying on more or less complex mechanical arrangements, which contributes to increasing the cost of the end product.
Small devices which can be adapted to the exterior of a battery whose resistance has a very high variation at a determined temperature, for example between 100xc2x0 C. and 110xc2x0 C., such as the devices proposed under the name xe2x80x9cPolyswitchxe2x80x9d by the Raychem company, have also been proposed by way of breaker switch. It is clear that such devices are sensitive to a nearby temperature but cannot react to the internal temperature of the battery. Even if these devices allow the battery to be used again after each incident, the resistance hysteresis phenomena have a very unfavourable effect on the reliability and lifespan of said battery. These xe2x80x9cpolyswitchesxe2x80x9d further have the drawback of having a resistance value which is close to the internal resistance value of the battery.
The object of the present invention is to overcome the aforementioned drawbacks by providing a breaker switch, sensitive to an abnormal rise in temperature integrated in a power supply or consumer device, and more particularly a breaker switch for a battery, of new design but sufficiently simple so as not to substantially increase the cost of the end product.
The invention therefore concerns a breaker switch arranged on at least one connection means between the outer terminals and an electric power supply or consumer device, contained in a container and whose internal operating temperature must not exceed a critical value xcex81. This breaker switch is arranged on a wall and in thermal communication with said container. It includes a receptacle intended to accommodate a thermofusible composition having a softening point xcex81, and a flexible metal strip interposed between two studs electrically breaking said connection means. The flexible strip extends above said receptacle while being fixed by one of its ends to the wall of the container and being provided in its free portion with anchoring means in said thermofusible composition. This flexible strip can occupy a first position called the closed position, in which the anchoring means are held in the thermofusible composition while its free portion is bent towards the wall of said container to establish an electric contact between the studs, and a second position called the open position in which it is released from the thermofusible composition by its elastic return force thereby opening the contact between the studs, when the temperature of the container is higher than melting temperature xcex81 of the thermofusible composition.
These anchoring means are formed by any means allowing a free portion of the strip and the receptacle containing the thermofusible composition to be fixed to each other while pressing said strip towards the container wall.
According to a first embodiment, these anchoring means are formed by a lug welded to the strip or formed therefrom.
According to a first variant, the anchoring lug is formed simply by bending back the free end of the strip. According to a second variant, the anchoring lug is attached to a median portion of the strip by welding, or is integral with the latter by a cut in a U shape which is open towards one of the ends of the strip. In this variant, the free portion situated beyond the lug allows an electric contact to be established with the stud.
In this first embodiment, the solidity of the setting of the lug in the thermofusible composition can be increased by providing additional anchoring means such as transverse cutting, folding a U shaped cut portion, roughness created or added on its surface, or simply by deforming its distal portion out of its plane.
According to a second embodiment, these anchoring means are created at the moment of use, by click fitting a pin, fixed in the thermofusible composition, in a cut out part formed in a free portion of the strip, said cut out part being for example in the shape of a cross.
In order to take account of the effective critical temperature of the device, the receptacle is advantageously extended by a heat probe which goes to the core of the container.
This type of breaker switch is particularly well suited to a battery, in particular a lithium-ion type battery which has the highest risk of malfunction. It can simply be added between a terminal and a conductor wire connected to a current collector. According to a preferred embodiment, the breaker switch includes a heat probe formed by a flattened metal bar or tube forming the anode or the cathode of the battery, the thermofusible composition receptacle being one of the studs of the connection means. A battery of this type can obviously also be provided with a safety vent of the type of those known in the prior art. According to a preferred embodiment, this safety vent is sensitive, not to an increase in the internal pressure of the battery, but to a rise in temperature and is formed by a passage passing through the cover and sealed by a thermofusible composition which is identical or different to that used for the breaker switch. According to another feature of the invention, the receptacle containing the thermofusible composition of the breaker switch is open towards the exterior and also forms the safety vent when the battery temperature exceeds a determined critical temperature. According to a variant which provides the battery with two levels of security, the sealing of the open receptacle is effected with two superposed thermofusible compositions: the first triggers the breaker switch when the temperature reaches the critical temperature and the second opens the vent when this temperature is exceeded by a certain number of degrees. It is possible to fix these two safety thresholds by a suitable selection from the thermofusible compositions available on the market, whose softening point can vary from 80xc2x0 C. to 150xc2x0 C.